The newer rocks, common also to the Nicobars and Sumatra, are in Ritchie's Archipelago chiefly and contain radiolarians and foraminifera.
Lime is, in fact, absorbed to an enormous extent by fishes, molluscs, crustacea, calcareous algae and sponges, starfishes, sea-urchins and feather stars, many polyzoa and a multitude of protozoa (mainly the foraminifera).
The proportion of calcium carbonate varies greatly according to the amount of foraminifera and other calcareous organisms which it contains.
The limestones contain Globigerina and other Foraminifera, the siliceous beds are made of Radiolaria, sponge spicules and diatoms, while the red clay closely resembles the red clay of the deepest parts of the oceans.
Green mud differs to a greater extent from the blue mud, and owes its characteristic nature and colour to the presence of glauconite, which is formed inside the cases of foraminifera, the spines of echini and the spicules of sponges in a manner not yet understood.